An organic electroluminescent element (hereinafter, also referred to as an organic EL element) has a configuration in which a light-emitting layer containing a light-emitting compound is sandwiched between an anode and a cathode. Furthermore, the organic electroluminescent element is a light-emitting element that generates an exciton by recombining a positive hole injected from an anode and an electron injected from a cathode in the light-emitting layer through application of an electric field, and that utilizes emission of light (fluorescence/phosphorescence) when the exciton is deactivated. The organic EL element is a complete solid state component in which a gap between electrodes is constituted of a film of an organic material having only an approximate submicron thickness, and can emit light with a voltage of approximately several volts to several tens of volts. Accordingly, the utilization for next-generation flat displays and flat lighting devices is expected.
There is proposed, as the organic EL element, for example, an element having a configuration in which two light-emitting layers in a complementary color relation to each other are laminated so as to make contact with each other to thereby achieve white light emission, namely, a so-called two-wavelength-type white light-emitting element. However, such two-wavelength-type white light-emitting element can give good white on the CIE chromaticity coordinate, but the emission spectrum thereof is not continuous and has only two peaks in a complementary color relation to each other. Accordingly, it is difficult to obtain broad white light close to natural light.
On the other hand, research and development of a white light-emitting element having an emission spectrum including each peak in respective wavelength regions of red, green and blue, namely, a so-called three-wavelength-type white light-emitting element, instead of the two-wavelength-type white light-emitting element, is also being advanced (see, for example, Patent Literature 1, Patent Literature 2).
As to three-wavelength-type white light-emitting elements described in Patent Literature 1 or Patent Literature 2, it is described that a high current efficiency (luminance obtained relative to a certain current density) can be obtained by laminating a plurality of light-emitting elements in series and superposing light emission from each of light-emitting elements. Furthermore, it is described that a white light-emitting element can be obtained by laminating light-emitting elements having different light emission colors in series.